Composite materials are increasingly used as substitutes for conventional materials such as aluminum and steel alloys in various structural components due to the generally high strength-to-weight ratio inherent in composite materials. Composite materials may generally be comprised of a network of reinforcing fibers that are generally applied in layers, and a polymeric resin that substantially wets the reinforcing fibers to form an intimate contact between the resin and the reinforcing fibers.
Composite materials exhibit modes of failure that are distinct from failure modes present in conventional materials. In particular, the material may deteriorate due to mechanical fatigue and/or environmental exposure so that the various layers in the composite material debond, forming delaminated regions within the material. In addition, the material may develop cracks and or other defects. In either case, the defect may not be detected by a visual inspection of the material. Accordingly, various non-invasive methods are available that may be used to locate defects in the composite material.
For example, in one commonly used method, a surface of the composite material is lightly and repeatedly tapped with a percussive device during the inspection, and the resulting sound is noted. If the sound resulting from the surface tapping has a relatively sharp report, the area is assumed to be generally free from internal defects. Correspondingly, if the resulting sound has a relatively hollow-sounding report, an internal defect may exist within the material at the location exhibiting the characteristic report. Although the foregoing method is simple to implement, and is suitable for a field inspection of the composite material, it relies on the subjective interpretation of sounds returned from the material, and may therefore be somewhat unreliable. In another related method, a small acoustic transducer is moved across the surface of the composite material so that an acoustic signal is projected into the material. Acoustic waves that are reflected from internal structures in the composite material are then processed to determine if internal structural anomalies exist. Although the subjectivity of the inspector is removed, the method requires a relatively sophisticated apparatus that may not be available for field use.
In another known method, a surface portion of the composite material is heated using a flash discharge lamp. After a predetermined delay period, the surface portion is imaged using an infrared camera to determine if an internal defect is present. The surface temperature decay during the delay period may then be used to infer the presence of an internal defect since debonding and/or delaminations generally cause localized and identifiable changes in the thermal conductivity of the material.
The foregoing methods generally require the availability of electrical power and therefore may be difficult to use in a field environment. Other drawbacks may also preclude the use of the foregoing methods in a field environment. For example, the use of flash lamps to provide a thermal heat input to the composite panel may not be possible in certain field environments. Accordingly, there is a need in the art for systems and methods for the thermographic inspection of composite materials that avoid the foregoing limitations.